The present invention is directed to printed circuit board lay-outs for decoupling electrical components mounted on the printed circuit boards, and methods of fabricating populated printed circuit boards having such lay-outs. The present invention is especially directed to such lay-outs, and such methods, for decoupling micro-ball grid array packaged electrical components (such as micro-ball grid array packaged chip sets) mounted on printed circuit boards, especially on motherboards. In particular, the present invention is directed to such lay-outs, and methods, whereby all decoupling caps (decoupling capacitors) can be provided on the front surface of the printed circuit board, while minimizing loop area of the decoupling circuit. The front surface of the printed circuit board is the surface on which the electrical components are mounted.
Previous lay-outs for decoupling circuits on printed circuit boards, such as motherboards, included designs wherein the decoupling caps populated both the front and rear surfaces of the motherboards. For example, previous lay-outs for decoupling MTH (82805AA), a chip set with a wire-bonded ball grid array package, made by Intel(copyright) Corp., required five decoupling caps placed on the backside (rear surface) of the motherboard, in addition to placing eight or nine decoupling caps on the top side (front surface) of the motherboard. Costs associated with placing decoupling caps on the rear surface of the motherboard are higher than those associated with placing decoupling caps on the front surface of the motherboard.
It has been proposed to provide lay-outs for, e.g., a motherboard, wherein all decoupling caps are on the front surface. In one such proposed technique, the circuit wiring between the micro-ball grid array packaged electrical component to be decoupled and the decoupling cap extends along the interior of the motherboard, and the effective current loop area, which directly translates to loop inductance, between the, e.g., micro-ball grid array packaged electrical component and decoupling cap, is disadvantageously large.